Power cylinders are well known in the prior art. Such power cylinders typically provide an enclosure having a piston and piston rod slidably disposed within the enclosure, wherein the piston rod extends outwardly from the cylinder. Pneumatic or hydraulic passageways are provided at each end of the cylinder, whereby pressurized fluid is supplied or exhausted on either side of the piston, thereby forcing the piston and piston rod to move from one end of the cylinder to the other. The piston and the piston rod move simultaneously to provide linear actuator movement to a workpiece. Through such movement, an actuation force is imparted on the workpiece as the piston rod moves between an extended position, wherein the piston rod extends outward from the cylinder, and a retracted position, wherein the piston is withdrawn within the cylinder.
The actuation forces applied to the workpiece are governed by the formula, F=PS, where F is the force, P is the pressure within the cylinder, and S is the surface area of the piston and/or piston rod perpendicular to the force component. When the piston rod moves from the retracted position to the extended position, the surface area S equals the sum of the  surface area of the piston and the end of the piston rod perpendicular to the force component. However, when the piston rod moves from the extended position to the retracted position, the surface area S equals only the surface area of the piston since the piston rod extends outside the cylinder and does not provide a surface area perpendicular to the force component or direction of movement. Therefore, the actuation forces generated in moving the workpiece from the retracted position to the extended position are typically greater than those generated when moving the workpiece from the extended position to the contracted position. The difference in the actuation forces is due to the difference in the surface areas of the piston and piston rod when moving between the extended and retracted positions.
It is sometimes desirable, however, to have different or similar actuation forces applied to the workpiece as the piston rod moves between the first position and the second position. For instance, it may be desirable to have the actuation forces greater or the same when moving toward the retracted position as opposed to the extended position. Previous designs have attempted to vary the actuation forces applied to the workpiece by manipulating the pressure levels within the power cylinder. These designs typically involve complex hydraulic and pneumatic controls and valves that are expensive to manufacture and difficult to maintain.
It would be desirable to provide a simple power cylinder that provides varying actuation forces to a workpiece. It would also be desirable to provide such a power cylinder that is inexpensive to manufacture and easy to maintain.